Groundwater is the moisture and water that exists in the spaces between rocks, the pores in the soil and fractures in the geology-the invisible portion of the water cycle. Groundwater is renewed through precipitation, which is often seasonal, but can be extracted year-round. Provided that there is adequate replenishment, and that the source is protected from pollution, groundwater can be extracted indefinitely and can be robust in the face of drought.
However, mankind is rarely prudent. Increase the amount of groundwater extracted, then
slowly over time the aquifer is used up. Development adds people and industry increasing
the demand for water while adding roads and buildings that prevent the
infiltration of precipitation into the ground. Essentially, reducing the recharge of the
aquifer while increasing the demand for water a potentially unsustainable
combination. Increase water use or reduce recharge by eliminating forested
areas and replacing with compacted soils (lawns that need to be watered),
pavement, buildings and over time the aquifer will become exhausted.
Groundwater is both used for water supply and serves to support steam flow between rain storms. Groundwater comes from rainwater and snow melt percolating into the ground. Typically, the deeper the well the further away is the water origination and the older the water. The groundwater age is a function of local geology, the amount of precipitation and the rate that water is pumped out of the aquifer. Geology also determines the ease with which water and contaminants can travel through an aquifer and the amount of water the land can hold. The land surface through which groundwater is recharged must remain open and uncontaminated to maintain the quality and quantity of groundwater.
The groundwater cycle in humid and arid regions differ fundamentally from each other. In humid climates, with high rainfall, large volumes of water seep into the groundwater, which contributes actively to the water cycle feeding streams, springs and wetlands during periods when the rainfall is lower. In semi-arid and arid climates, there is by contrast practically no exchange between the surface water and groundwater because the small volume of seepage from the occasional rainfall only rarely penetrates the thick and dry (unsaturated) soils. That groundwater tends to be much deeper and isolated from surface contact. In currently arid areas groundwater resources are only minimally recharged. Our understanding of the complete water cycle is still only rudimentary.
We do know that groundwater availability varies by location. Precipitation and soil type determines how much the shallower groundwater is recharged annually. However the volume of water that can be stored is controlled by the reservoir characteristics of the subsurface rocks. Water resources can be used sustainably only if their volume and variation through time are understood. However such information is often lacking. Hydrology as a science is very young and so little is known. Any attempt to accurately model the groundwater component of the water cycle requires adequate measurements and observations over decades. The computer models in common use in the United States only address the shallower groundwater and surface water interactions and tend to assume linear relationship which scientists are finding is not accurate.
Groundwater is usually cleaner than surface water. Groundwater is typically protected against contamination from the surface by the soils and rock layers covering the aquifer. This is the only available clean drinking water in many parts of the world. However, rising world population, changes in land use and rapid industrialization are increasingly place groundwater in jeopardy. Once contaminated, groundwater is very difficult to clean and often after removal of contaminated plumes only long term abandonment of use to allow for natural attenuation is the only possible course of action. As droughts and water shortages appear the value of groundwater has begun to be more fully appreciated. Precious groundwater resources increasingly need to be protected and well managed to allow for sustainable long-term use.
Water-table aquifers are usually shallower than confined
aquifers and because they are shallow, they are impacted by drought conditions and
surface contaminants more easily than confined aquifers. Thus, most public
supply water wells draw from the deeper confined aquifers. The water is drawn
from the fine-grained confining layers called aquitards. Water enters these
aquitards very, very slowly and the danger in utilizing them for supply is that
they become overdrawn. When that happens
an irreversible compaction of the fined-grained confining layer occurs and
there is permanent subsidence. The land surface falls, permanently reducing the
storage capacity of underground aquifers, threatening future water supplies.
The demand for water is rising as population, economic activity and agricultural irrigation grow. We need to manage our water resources in a sustainable way if we are to have a future. To survive over time, a population must live within its available resources. Water is essential for life. We need water for drinking, bathing, irrigated agriculture and industry. A large portion of the fresh water on earth is groundwater. It moves and changes over time and there limits to the amount of groundwater available for extraction from an aquifer. To be sustainable, the amount of groundwater removed from an aquifer needs to match the recharge rate.